PURPOSE: To allow automatic detection of zero motor speed by providing a zero speed decision circuit which makes a decision that the rotational speed is zero based on a motor stop signal received from a bearing vibration detecting circuit and a circuit breaker open signal received from a (b) contact circuit and produces a motor start permission signal.

CONSTITUTION: The circuit breaker throw-in circuit device comprises a bearing vibration measuring sensor 5 for detecting vibration of bearing in a motor 4 and producing a signal corresponding to the amplitude value, a bearing vibration detecting circuit 6 for receiving a signal from the sensor 5 and detecting vibration, and a zero speed decision circuit 8 which makes a decision that the speed of the motor 4 is zero based on a zero amplitude value. Upon turn OFF of a circuit breaker 1 for motor circuit, a (b) contact circuit 7 closes to turn the zero speed decision circuit 8 ON and when the bearing vibration value goes zero, the zero speed decision circuit 8 makes a decision that the motor 4 stopped. Upon elapse of a predetermined time, a contact 9 in a motor start interlock circuit is turned ON to release interlock of a circuit breaker throw-in coil 3 automatically.